1. Field of the Invention
The present invention relates to a constant current power supply apparatus for driving a load with a constant current, and particularly relates to a constant current power supply apparatus for driving a load with a periodic pulse signal under PWM (Pulse Width Modulation) control.
2. Description of the Related Art
The LED (Light Emitting Diode) has a feature that it changes the color tone in response to the magnitude of a current. Therefore, in the case where an LED is driven as a load while performing dimming control, the LED is generally driven on/off with a pulse signal under PWM control for adjusting the quantity of light by the duty ratio for the pulse signal.
On the other hand, in the case where a switching power supply is used as a constant current power supply apparatus for driving the load with a constant current, it is necessary to detect the output current for performing the feedback control. In the case where an LED is driven as a load under PWM control as described above, the LED will repeat the turn-on period and the turn-off period, and understandably, in the turn-off period, no current is passed through the LED, resulting in the output current being fed back as a zero current. If the output current is thus fed back as a zero current, too much electric power is supplied; then in order to prevent an excessive amount of electricity from being supplied, it has been proposed to perform the feedback control only in the period during which the LED is turned on (for example, Patent Documents 1 and 2 to be referenced).
With Patent Document 1, when the N-type MOS transistor for turning the LED on/off is on, the electric power is supplied from the switching power supply to the LED, and the output current flowing through the LED is detected for performing the constant current control. In addition, when the N-type MOS transistor is off, the supply of electric power from the switching power supply to the LED is halted.
With Patent Document 2, the period during which a current flows in the LED and the period during which no current flows in the LED are used to alter the minimum off-period for a switching device in the switching power supply. Specifically, the minimum off-period is set at such a length that, in the period during which no current flows in the LED, the switching device is fully off for halting the supply of electric power to the LED.